Pokemon Yellow Virtual Console save not loading in PKHeX (CFW 3ds, dumped with Checkpoint)

[Lokii270]

Title pretty much.
I've got a Virtual Console release of Pokemon yellow with about 20 hours gameplay, no issues.
I've backed up the save with Checkpoint then moved it to my PC. Tried loading the sav.dat into PKHeX but I get the unsupported file type/size error. Tried a few times and I'm getting the same error. What am I doing wrong thanks?

@theSLAYER Specifically hoping you can help me!

[Lokii270]

sav.dat

[Kaphotics]

Current box is invalid. There's nothing in your boxes?

[Lokii270]

That's true there are no Pokemon in my boxes. Is that a requirement for PKHeX to load the save?

[Lokii270]

Would love some help with this

[theSLAYER]

Could you place pokemon in your box first?

[Lokii270]

The save loads with a Pokemon in the box. Didn't realise this was a requirement. Thankyou